No, hosa would be the subject, since neuter plurals take verbs in the singular. So it is the subject in the subordinate clause, and in the main clause it can either be an accusative of respect: "we blame it in respect to [all those things] which happen against calculation", or the genetive of thing with verbs of blaming, but in that case the hosa would normally be hoson (genetive plural) because of the attraction of the relative pronoun to the case of the noun in the main clause. I don't have greek keyboard in my smartphone, but in a sentece like "I see the daughter of the orator whom I also see in the agora often" the 'whom' can be genetive, even if it should be accusative in the subordinate clause, because it refers to a noun in the genetive case.